Electronic devices with capacitive proximity sensors for proximity-based radio-frequency power control

ABSTRACT

An electronic device may have a housing in which an antenna is mounted. An antenna window may be mounted in the housing to allow radio-frequency signals to be transmitted from the antenna and to allow the antenna to receive radio-frequency signals. Near-field radiation limits may be satisfied by reducing transmit power when an external object is detected in the vicinity of the dielectric antenna window and the antenna. A capacitive proximity sensor may be used in detecting external objects in the vicinity of the antenna. The proximity sensor may have conductive layers separated by a dielectric. A capacitance-to-digital converter may be coupled to the proximity sensor by inductors. The capacitive proximity sensor may be interposed between an antenna resonating element and the antenna window. The capacitive proximity sensor may serve as a parasitic antenna resonating element and may be coupled to the housing by a capacitor.

This application claims the benefit of provisional patent applicationNo. 61/226,683, filed Jul. 17, 2009, which is hereby incorporated byreference herein in its entirety.

BACKGROUND

This relates generally to antennas, and, more particularly, to antennasfor electronic devices.

Electronic devices such as portable computers and handheld electronicdevices are becoming increasingly popular. Devices such as these areoften provided with wireless communications capabilities. For example,electronic devices may use long-range wireless communications circuitrysuch as cellular telephone circuitry to communicate using cellulartelephone bands at 850 MHz, 900 MHz, 1800 MHz, and 1900 MHz (e.g., themain Global System for Mobile Communications or GSM cellular telephonebands). Long-range wireless communications circuitry may also be usedhandle the 2100 MHz band and other bands. Electronic devices may useshort-range wireless communications links to handle communications withnearby equipment. For example, electronic devices may communicate usingthe WiFi® (IEEE 802.11) bands at 2.4 GHz and 5 GHz (sometimes referredto as local area network bands) and the Bluetooth® band at 2.4 GHz.

It can be difficult to incorporate antennas successfully into anelectronic device. Some electronic devices are manufactured with smallform factors, so space for antennas is limited. In many electronicdevices, the presence of electronic components in the vicinity of anantenna serves as a possible source of electromagnetic interference.Antenna operation can also be blocked by conductive structures. This canmake it difficult to implement an antenna in an electronic device thatcontains conductive housing walls or other conductive structures thatcan potentially block radio-frequency signals. Radio-frequency transmitpower limits may be imposed by regulatory bodies. These limits posechallenges when operating an electronic device antenna at elevated powerlevels.

It would therefore be desirable to be able to provide improved antennasfor wireless electronic devices.

SUMMARY

An electronic device such as a tablet computer or other portable devicemay have a conductive housing. A portion of the conductive housing ineach device may serve as antenna ground for an antenna. The antenna maybe fed using a positive antenna feed terminal coupled to an antennaresonating element and a ground antenna feed terminal coupled to theconductive housing.

The antenna resonating element may be mounted adjacent to an antennawindow in the conductive housing. To ensure that desired maximum outputpower limits for radio-frequency signals are satisfied when an externalobject such as a human body is in the vicinity of the antenna window,the electronic device may be provided with a capacitive proximitysensor. The proximity sensor may have a capacitive proximity sensorelectrode that is interposed between the antenna resonating element andthe antenna window. During operation, the proximity sensor may detectwhen an external object such as part of a user's body comes within agiven distance of the proximity sensor and the antenna. When theseconditions are detected, circuitry in the electronic device may reducethe maximum transmitted output power through the antenna.

The capacitive proximity sensor electrode may have first and secondconductive layers that are separated by a dielectric layer. First andsecond inductors may be used to couple the first and second conductivelayers to respective first and second inputs of a signal detector suchas a capacitance-to-digital converter.

The capacitive proximity sensor electrode may serve as a parasiticantenna resonating element for the antenna that helps to reduceradio-frequency signal hotspots. A capacitor may be used to connect thecapacitive proximity sensor electrode to the conductive housing.

Further features of the invention, its nature and various advantageswill be more apparent from the accompanying drawings and the followingdetailed description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front perspective view of an illustrative electronic devicewith an antenna and proximity sensor in accordance with an embodiment ofthe present invention.

FIG. 2 is a rear perspective view of an illustrative electronic devicewith an antenna and proximity sensor in accordance with an embodiment ofthe present invention.

FIG. 3 is a schematic diagram of an illustrative electronic device withantenna and proximity sensor structures in accordance with an embodimentof the present invention.

FIG. 4 is a cross-sectional side view of an illustrative electronicdevice with an antenna and proximity sensor in accordance with anembodiment of the present invention.

FIG. 5 is a diagram of an illustrative electronic device having anantenna and wireless circuitry that may reduce the amount of powertransmitted through the antenna when a proximity sensor detects that anexternal object is within a given range of the antenna and theelectronic device in accordance with an embodiment of the presentinvention.

FIG. 6 is a perspective view of an illustrative antenna having anantenna resonating element and a proximity sensor electrode serving as aparasitic antenna resonating element that overlap a dielectric antennawindow in accordance with an embodiment of the present invention.

FIG. 7 is a graph showing how the presence of a parasitic antennaresonating element may help to reduce radio-frequency signal hotspotsand thereby reduce near field radiation hotspots produced by an antennain an electronic device in accordance with an embodiment of the presentinvention.

FIG. 8 is a top view of a parasitic antenna resonating element such as acapacitive proximity sensor electrode that has been coupled by acapacitor to a portion of a conductive device housing that is serving asantenna ground in accordance with an embodiment of the presentinvention.

FIG. 9 is a diagram showing how a proximity sensor may have a capacitorelectrode for detecting the presence of external objects such as a partof a user's body in accordance with an embodiment of the presentinvention.

FIG. 10 is a diagram showing how a capacitive proximity sensor may havea two-layer capacitive sensor having a shield electrode and a sensorelectrode that are monitored by a capacitance-to-digital converter inaccordance with an embodiment of the present invention.

FIG. 11 is a perspective view of an illustrative two-layer capacitiveproximity sensor electrode structure in accordance with an embodiment ofthe present invention.

FIG. 12 is a perspective view of an elongated two-layer capacitiveproximity sensor electrode in accordance with an embodiment of thepresent invention.

DETAILED DESCRIPTION

Electronic devices may be provided with wireless communicationscircuitry. The wireless communications circuitry may be used to supportwireless communications in one or more wireless communications bands.For example, the wireless communications circuitry may transmit andreceive signals in cellular telephone bands.

To satisfy consumer demand for small form factor wireless devices,manufacturers are continually striving to reduce the size of componentsthat are used in these devices while providing enhanced functionality.Particularly in configurations in which an electronic device is used intransmitting and receiving radio-frequency signals in cellular telephonebands and other communications bands that have relatively widebandwidths, it can be challenging to meet desired antenna performancecriteria in a compact device. High transmit powers and wide antennabandwidths can be desirable to ensure adequate signal strength duringcommunications, but these attributes may give rise to challenges withcontrolling emitted radiation levels.

It is generally impractical to completely shield a user of an electronicdevice from transmitted radio-frequency signals. For example,conventional cellular telephone handsets generally emit signals in thevicinity of a user's head during telephone calls. Government regulationslimit radio-frequency signal powers. At the same time, wireless carriersrequire that the user equipment that is used in their networks becapable of producing certain minimum radio-frequency powers so as toensure satisfactory operation of the equipment.

In many jurisdictions, specific absorption rate (SAR) standards are inplace that impose maximum energy absorption limits on handsetmanufacturers. These standards place restrictions on the amount ofradiation that may be emitted at any particular point within a givendistance of the antenna. Particular attention is given to radiationlimits at distances of about 1-20 mm from the device, where users arelikely to place a body part near an antenna.

Satisfactory antenna performance and regulatory compliance can beensured by using an antenna does not exhibit local “hotspots” in whichemitted radiation exceeds desired power levels. A proximity sensor mayalso be used to detect when an external object such as a user's body isin the vicinity of the antenna. When the presence of the external objectis detected, transmitted power levels can be reduced.

Hotspots can be minimized by proper antenna design. If desired, aparasitic antenna resonating element may be placed in the vicinity of adevice antenna to help smooth out near-field emitted radiation patterns.Electromagnetic shielding arrangements may also be implemented usingferrite tape or other high permeability materials.

Any suitable electronic devices may be provided with antennas andproximity sensors that use these configurations. As an example, antennasand proximity sensors may be formed in electronic devices such asdesktop computers, portable computers such as laptop computers andtablet computers, handheld electronic devices such as cellulartelephones, etc. With one suitable configuration, which is sometimesdescribed herein as an example, the antennas and proximity sensors areformed in relatively compact electronic devices in which interior spacecan be valuable. These compact devices may be portable electronicdevices.

Portable electronic devices that may be provided with antennas andproximity sensors include laptop computers and small portable computerssuch as ultraportable computers, netbook computers, and tabletcomputers. Portable electronic devices may also be somewhat smallerdevices. Examples of smaller portable electronic devices that may beprovided with antennas include cellular telephones, wrist-watch devices,pendant devices, headphone and earpiece devices, and other wearable andminiature devices.

Space is at a premium in portable electronic devices and housings forthese devices are sometimes constructed from conductive materials thatblock antenna signals. Arrangements in which antenna structures andproximity sensors are formed behind an antenna window can help addressthese challenges. Antenna windows may be formed in conductive housingwalls by forming a dielectric antenna window structure from an openingin the conductive housing wall. If desired, slot-based antenna windowsmay be formed in conductive housing walls. In a slot-based antennawindow, the window region is defined by a pattern of window slots.Arrangements in which dielectric antenna windows are used are sometimesdescribed herein as an example.

An antenna resonating element may be formed under the antenna window.Portions of the conductive housing or other conductive structures mayserve as antenna ground. The antenna can be fed using a positive antennafeed terminal that is coupled to the antenna resonating element andusing a ground antenna feed terminal that is coupled to the conductivehousing. During operation, radio-frequency signals for the antenna canpass through the antenna window. The parasitic antenna resonatingelement and ferrite tape may help to reduce near-field hotspots.

A proximity-based antenna power control circuit may be used to reducenear-field electromagnetic radiation intensities when the presence of anexternal object is detected in the vicinity of the antenna. Theproximity-based antenna power control circuit may be based on acapacitive proximity sensor. Sensor electrodes for the capacitiveproximity sensor may be placed in the vicinity of the antenna. Ifdesired, a conductive structure such as a sensor electrode may serveboth as part of a capacitive sensor and as part of a parasitic antennaresonating element. With this type of arrangement, the sensor electrodemay be used in reducing near-field radiation hotspots whilesimultaneously serving as part of a capacitor electrode that detects thepresence of nearby external objects for a proximity detector.

Antenna and proximity sensor structures with configurations such asthese can be mounted on any suitable exposed portion of a portableelectronic device. For example, antennas and proximity sensors can beprovided on the front or top surface of the device. In a tabletcomputer, cellular telephone, or other device in which the front of thedevice is all or mostly occupied with conductive structures such as atouch screen display, it may be desirable to form at least part of anantenna window on a rear device surface. Other configurations are alsopossible (e.g., with antennas and proximity sensors mounted in moreconfined locations, on device sidewalls, etc.). The use of antennamounting locations in which at least part of a dielectric antenna windowis formed in a conductive rear housing surface is sometimes describedherein as an example, but, in general, any suitable antenna mountinglocation may be used in an electronic device if desired.

An illustrative portable device that may include an antenna andproximity sensor is shown in FIG. 1. As shown in FIG. 1, device 10 maybe a relatively thin device such as a tablet computer. Device 10 mayhave display such as display 50 mounted on its front (top) surface.Housing 12 may have curved portions that form the edges of device 10 anda relatively planar portion that forms the rear surface of device 10 (asan example). An antenna window such as antenna window 58 may be formedin housing 12. Antenna structures for device 10 may be formed in thevicinity of antenna window 58.

Device 10 may have user input-output devices such as button 59. Display50 may be a touch screen display that is used in gathering user touchinput. The surface of display 50 may be covered using a dielectricmember such as a planar cover glass member. The central portion ofdisplay 50 (shown as region 56 in FIG. 1) may be an active region thatis sensitive to touch input. The peripheral regions of display 50 suchas regions 54 may be inactive regions that are free from touch sensorelectrodes. A layer of material such as an opaque ink may be placed onthe underside of display 50 in peripheral regions 54 (e.g., on theunderside of the cover glass). This layer may be transparent toradio-frequency signals. The conductive touch sensor electrodes inregion 56 may tend to block radio-frequency signals. However,radio-frequency signals may pass through the cover glass and opaque inkin inactive display regions (as an example). In the opposite direction,radio-frequency signals may pass through antenna window 58.Lower-frequency electromagnetic fields also pass through window 58, socapacitance measurements for a proximity sensor may be made throughantenna window 58.

Housing 12 may be formed from one or more structures. For example,housing 12 may include an internal frame and planar housing walls thatare mounted to the frame. Housing 12 may also be formed from a unitaryblock of material such as a cast or machined block of aluminum.Arrangements that use both of these approaches may also be used ifdesired.

Housing 12 may be formed of any suitable materials including plastic,wood, glass, ceramics, metal, or other suitable materials, or acombination of these materials. In some situations, portions of housing12 may be formed from a dielectric or other low-conductivity material,so as not to disturb the operation of conductive antenna elements thatare located in proximity to housing 12. In other situations, housing 12may be formed from metal elements. An advantage of forming housing 12from metal or other structurally sound conductive materials is that thismay improve device aesthetics and may help improve durability andportability.

With one suitable arrangement, housing 12 may be formed from a metalsuch as aluminum. Portions of housing 12 in the vicinity of antennawindow 58 may be used as antenna ground. Antenna window 58 may be formedfrom a dielectric material such as polycarbonate (PC), acrylonitrilebutadiene styrene (ABS), a PC/ABS blend, or other plastics (asexamples). Window 58 may be attached to housing 12 using adhesive,fasteners, or other suitable attachment mechanisms. To ensure thatdevice 10 has an attractive appearance, it may be desirable to formwindow 58 so that the exterior surfaces of window 58 conform to the edgeprofile exhibited by housing 12 in other portions of device 10. Forexample, if housing 12 has straight edges 12A and a flat bottom surface,window 58 may be formed with a right-angle bend and vertical sidewalls.If housing 12 has curved edges 12A, window 58 may have a similarlycurved surface.

FIG. 2 is a rear perspective view of device 10 of FIG. 1 showing howdevice 10 may have a relatively planar rear surface 12B and showing howantenna window 58 may be rectangular in shape with curved portions thatmatch the shape of curved housing edges 12A.

A schematic diagram of device 10 showing how device 10 may include oneor more antennas 26 and transceiver circuits that communicate withantennas 26 is shown in FIG. 3. Electronic device 10 of FIG. 3 may be aportable computer such as a laptop computer, a portable tablet computer,a mobile telephone, a mobile telephone with media player capabilities, ahandheld computer, a remote control, a game player, a global positioningsystem (GPS) device, a desktop computer, a combination of such devices,or any other suitable electronic device.

As shown in FIG. 3, electronic device 10 may include storage andprocessing circuitry 16. Storage and processing circuitry 16 may includeone or more different types of storage such as hard disk drive storage,nonvolatile memory (e.g., flash memory or otherelectrically-programmable-read-only memory), volatile memory (e.g.,static or dynamic random-access-memory), etc. Processing circuitry instorage and processing circuitry 16 may be used to control the operationof device 10. Processing circuitry 16 may be based on a processor suchas a microprocessor and other suitable integrated circuits. With onesuitable arrangement, storage and processing circuitry 16 may be used torun software on device 10, such as internet browsing applications,voice-over-internet-protocol (VOIP) telephone call applications, emailapplications, media playback applications, operating system functions,control functions for controlling radio-frequency power amplifiers andother radio-frequency transceiver circuitry, etc. Storage and processingcircuitry 16 may be used in implementing suitable communicationsprotocols. Communications protocols that may be implemented usingstorage and processing circuitry 16 include internet protocols, cellulartelephone protocols, wireless local area network protocols (e.g., IEEE802.11 protocols—sometimes referred to as WiFi®), protocols for othershort-range wireless communications links such as the Bluetooth®protocol, etc.

Input-output circuitry 14 may be used to allow data to be supplied todevice 10 and to allow data to be provided from device 10 to externaldevices. Input-output devices 18 such as touch screens and other userinput interface are examples of input-output circuitry 14. Input-outputdevices 18 may also include user input-output devices such as buttons,joysticks, click wheels, scrolling wheels, touch pads, key pads,keyboards, microphones, cameras, etc. A user can control the operationof device 10 by supplying commands through such user input devices.Display and audio devices may be included in devices 18 such asliquid-crystal display (LCD) screens, light-emitting diodes (LEDs),organic light-emitting diodes (OLEDs), and other components that presentvisual information and status data. Display and audio components ininput-output devices 18 may also include audio equipment such asspeakers and other devices for creating sound. If desired, input-outputdevices 18 may contain audio-video interface equipment such as jacks andother connectors for external headphones and monitors.

Wireless communications circuitry 20 may include radio-frequency (RF)transceiver circuitry 23 formed from one or more integrated circuits,power amplifier circuitry, low-noise input amplifiers, passive RFcomponents, one or more antennas, and other circuitry for handling RFwireless signals. Wireless signals can also be sent using light (e.g.,using infrared communications).

Wireless communications circuitry 20 may include radio-frequencytransceiver circuits for handling multiple radio-frequencycommunications bands. For example, circuitry 20 may include transceivercircuitry 22 that handles 2.4 GHz and 5 GHz bands for WiFi (IEEE 802.11)communications and the 2.4 GHz Bluetooth communications band. Circuitry20 may also include cellular telephone transceiver circuitry 24 forhandling wireless communications in cellular telephone bands such as theGSM bands at 850 MHz, 900 MHz, 1800 MHz, and 1900 MHz, and the 2100 MHzdata band (as examples). Wireless communications circuitry 20 caninclude circuitry for other short-range and long-range wireless links ifdesired. For example, wireless communications circuitry 20 may includeglobal positioning system (GPS) receiver equipment, wireless circuitryfor receiving radio and television signals, paging circuits, etc. InWiFi and Bluetooth links and other short-range wireless links, wirelesssignals are typically used to convey data over tens or hundreds of feet.In cellular telephone links and other long-range links, wireless signalsare typically used to convey data over thousands of feet or miles.

Wireless communications circuitry 20 may include antennas 26 such as theantenna located adjacent to antenna window 58 of FIGS. 1 and 2. Antennas26 may be single band antennas that each cover a particular desiredcommunications band or may be multiband antennas. A multiband antennamay be used, for example, to cover multiple cellular telephonecommunications bands. If desired, a dual band antenna may be used tocover two WiFi bands (e.g., 2.4 GHz and 5 GHz). Different types ofantennas may be used for different bands and combinations of bands. Forexample, it may be desirable to form a dual band antenna for forming alocal wireless link antenna, a multiband antenna for handling cellulartelephone communications bands, and a single band antenna for forming aglobal positioning system antenna (as examples).

Transmission line paths 44 may be used to convey radio-frequency signalsbetween transceivers 22 and 24 and antennas 26. Radio-frequencytransceivers such as radio-frequency transceivers 22 and 24 may beimplemented using one or more integrated circuits and associatedcomponents (e.g., switching circuits, matching network components suchas discrete inductors, capacitors, and resistors, and integrated circuitfilter networks, etc.). These devices may be mounted on any suitablemounting structures. With one suitable arrangement, transceiverintegrated circuits may be mounted on a printed circuit board. Paths 44may be used to interconnect the transceiver integrated circuits andother components on the printed circuit board with antenna structures indevice 10. Paths 44 may include any suitable conductive pathways overwhich radio-frequency signals may be conveyed including transmissionline path structures such as coaxial cables, microstrip transmissionlines, etc.

Antennas 26 may, in general, be formed using any suitable antenna types.Examples of suitable antenna types for antennas 26 include antennas withresonating elements that are formed from patch antenna structures,inverted-F antenna structures, closed and open slot antenna structures,loop antenna structures, monopoles, dipoles, planar inverted-F antennastructures, hybrids of these designs, etc. With one suitablearrangement, which is sometimes described herein as an example, part ofhousing 12 (e.g., the portion of housing 12 in the vicinity of antennawindow 58) may form a ground structure for the antenna associated withwindow 58.

A cross-sectional view of device 10 in the vicinity of antenna window 58is shown in FIG. 4. As shown in FIG. 4, antenna 26 may have antennaresonating element 68 (e.g., a patch antenna resonating element, asingle arm inverted-F antenna structure, a dual-arm inverted-F antennastructure, or other suitable multi-arm or single arm inverted-F antennastructures, closed and open slot antenna structures, loop antennastructures, monopoles, dipoles, planar inverted-F antenna structures,hybrids of these designs, etc. Housing 12 may serve as antenna groundfor antenna 26.

Antenna 26 may also have a parasitic antenna resonating element formedfrom one or more conductive structures such as structure 66. Structure66 may include, for example, a capacitive proximity sensor electrode. Ifdesired, a layer of ferrite material such as ferrite tape 74 may beplaced between antenna resonating element 68 and window 58 to helpreduce near-field signal strengths without over-attenuating far-fieldsignals. In the example of FIG. 4, ferrite tape 74 has been placed understructure 66.

As shown in FIG. 4, antenna 26 may be fed using a positive antenna feedterminal that is coupled to antenna resonating element 68 such aspositive antenna feed terminal 76 and a ground antenna feed terminalthat is coupled to housing 12 such as ground antenna feed terminal 78.

Antenna resonating element 68 may be placed in the vicinity ofdielectric antenna window 58 as shown in FIG. 4, so that radio-frequencysignals can be conveyed through window 58 (e.g., in directions 72 and71). Radio-frequency signals can also be conveyed through a transparentdisplay cover member such as cover glass 60. Display 50 may have anactive region such as region 56 in which cover glass 60 has underlyingconductive structure such as display panel module 64. The structures indisplay panel 64 such as touch sensor electrodes and active displaypixel circuitry may be conductive and may therefore attenuateradio-frequency signals. In region 54, however, display 50 may beinactive (i.e., panel 64 may be absent). An opaque ink such as ink 62may be formed on the underside of transparent cover glass 60 in region54 to block antenna resonating element 68 from view. Ink 62 and thedielectric material of cover member 60 in region 54 may be sufficientlytransparent to radio-frequency signals that radio-frequency signals canbe conveyed through these structures in directions 70.

Any suitable conductive materials may be used in forming antennastructures for antenna 26. With one suitable arrangement, the conductivestructures for antenna resonating element 68 and parasitic antennaresonating element 66 may each be formed from conductive traces on adielectric support. The conductive traces may be formed from copper orother metals (as an example) to help ensure low losses and goodperformance at radio frequencies. The dielectric supports for thesestructures may be printed circuit boards or plastic members. Plasticsupport structures may also be used to support printed circuit boards.In general, printed circuit boards may be rigid or flexible. Rigidprinted circuit boards may be formed from epoxy (e.g., FR4) or otherdielectric substrates. Flexible printed circuit boards (“flex circuits”)may be formed from flexible polymer sheets such as polyimide sheets orother flexible dielectrics. When an antenna structure is formed from asheet of flex circuit substrate, the flex circuit may, if desired, beflexed to form a curved surface (e.g., to adapt to a curved plasticsupport structure). With rigid substrate arrangements, the printedcircuit board is typically flat.

Structures such as conductive structure 66 may serve multiple functions.For example, because structure 66 is adjacent to antenna resonatingelement 68, structure 66 influences the electromagnetic behavior ofantenna 26 and can therefore serve as a parasitic antenna resonatingelement. At the same time, conductive structure 66 may, if desired, beused as a sensor electrode for a proximity sensor.

Transceiver circuitry 23 may be mounted to printed circuit board 79 andmay be connected to the conductive lines in transmission line 44 viaconnector 81 and traces in board 79. Transmission line 44 may havepositive and ground conductors and may be used in conveyingradio-frequency antenna signals between transceiver 23 and feedterminals 76 and 78 of antenna 26.

Device 10 and antenna window 58 may have any suitable dimensions. Forexample, device 10 may have lateral dimensions of about 10-50 cm. Device10 may be more than 2 cm thick, less than 2 cm thick, less than 1.5 cmthick, or less than 0.5 cm thick.

In thin device configurations, the removal of conductive housingportions in the immediate vicinity of antenna resonating element 68helps ensure that antenna 26 will exhibit satisfactory efficiency andbandwidth (e.g., for supporting communications in wide bandwidthlong-range communications bands such as cellular telephonecommunications bands).

A circuit diagram showing how a proximity sensor signal may be used incontrolling the amount of power that is transmitted by antenna 26 isshown in FIG. 5. As shown in FIG. 5, device 10 may include storage andprocessing circuitry 16 (see, e.g., FIG. 3). Device 10 may also includea proximity sensor such as proximity sensor 80. Proximity sensor 80 maybe implemented using any suitable type of proximity sensor technology(e.g., capacitive, optical, etc.). An advantage of capacitive proximitysensing techniques is that they can be relatively insensitive to changesin the reflectivity of external object 87.

As shown in the example of FIG. 5, proximity sensor 80 may contain acapacitor electrode formed from a conductive member such as conductivemember 66 (FIG. 4). Conductive member 66 may, if desired, serve as aparasitic antenna resonating element for antenna 26.

Proximity sensor 80 may be mounted in housing 12 in the vicinity ofantenna 26 (as shown in FIG. 4) to ensure that the signal from proximitysensor 80 is representative of the presence of external object 87 in thevicinity of antenna 26 (e.g., within a distance D of antenna 26 and/ordevice 10).

Output signals from proximity sensor 80 may be conveyed to storage andprocessing circuitry 16 using path 86. The signals from proximity sensor80 may be analog or digital signals that provide proximity data tostorage and processing circuitry 16. The proximity data may be Booleandata indicating that object 87 is or is not within a given predetermineddistance of antenna 26 or may be continuous data representing a currentestimated distance value for D.

Storage and processing circuitry 16 may be coupled to transceivercircuitry 23 and power amplifier circuitry 82. Dashed line 83 shows howreceived radio-frequency signals can be conveyed from antenna 26 totransceiver circuitry 23. During data transmission operations, controllines 84 may be used to convey control signals from storage andprocessing circuitry 16 to transceiver circuitry 23 and power amplifiercircuitry 82 to adjust output powers in real time. For example, whendata is being transmitted, transceiver 23 and is associated outputamplifier 82 can be directed to increase or decrease the power level ofthe radio-frequency signal that is being provided to antenna 26 overtransmission line 44 to ensure that regulatory limits forelectromagnetic radiation emission are satisfied. If, for example,proximity sensor 80 does not detect the presence of external object 87,power can be provided at a relatively high (unrestricted) level. If,however, proximity sensor 80 determines that the user's leg or otherbody part or other external object 87 is in the immediate vicinity ofantenna 26 (e.g., within 20 mm or less, within 15 mm or less, within 10mm or less, etc.), storage and processing circuitry can respondaccordingly by directing transceiver circuitry 23 and/or power amplifier82 to transmit radio-frequency signals through antenna 26 at reducedpowers.

A perspective view of an illustrative antenna 26 is shown in FIG. 6. Asshown in FIG. 6, antenna resonating element 68 may contain one or moreconductive traces such as conductive trace 96. In the example of FIG. 6,antenna resonating element 68 has an inverted-F configuration. With thisconfiguration, antenna resonating element 68 may have a dielectricsubstrate such as rigid or flexible printed circuit substrate 90 onwhich a conductive pattern has been formed such as conductive trace 94.Conductive trace 94 may have a main resonating element arm 92, a shortcircuit branch such as branch 96 that shorts arm 92 to ground (e.g., apath coupled to antenna feed terminal 78 of FIG. 4), and a branch 98 towhich positive antenna feed terminal 76 is coupled. Arm 92 may, ifdesired, be provided with different shapes (e.g., multiple branches) tosupport operation in desired communications bands with desiredbandwidths. The trace pattern for antenna resonating element 68 that isshown in FIG. 6 is merely illustrative. In general, any suitable type ofantenna resonating element pattern may be used for antenna resonatingelement 68 if desired.

Antenna resonating element 68 may be mounted so as to overlap antennawindow 58 and so as to lie under inactive region 54 of display 50 (FIG.4). Conductive structure 66 may be interposed between antenna resonatingelement 68 and window 58.

During operation of antenna 26, the electromagnetic fields that areproduced by antenna resonating element 68 may induce currents inconductive housing 12, such as currents 95 in the vicinity of window 58.If care is not taken, the relative shapes and sizes of the components ofantenna 26 may give rise to undesirable concentrations of currents. Thiscan, in turn, lead to undesirable hotspots in the near-field radiationpattern for antenna 26, as the induced currents re-radiateelectromagnetic energy through antenna window 58.

A graph that illustrates how antenna signals may exhibit undesirablehotspots is shown in FIG. 7. In the graph of FIG. 7, the powerassociated with near-field transmitted radio-frequency signals (e.g.,signals for an antenna 26 that have been emitted in direction 72 or 71through antenna window 58) is shown as a function of position (e.g.,position along the inner edge of antenna window 58). Solid line 120corresponds to a possible near-field radiation pattern in the absence ofsuitable antenna structures to reduce hotspots in currents 95 andassociated hotspots in emitted radio-frequency signal powers. Dashedline 122 shows how hotspots can minimized or eliminated by inclusion ofproper hotspot-reducing structures. Because dashed line 122 is smootherthan line 120 and exhibits lower peak powers, dashed line 122 reflects areduced spatial concentration of radio-frequency signal power. Smoothedradiation characteristics help antenna 26 to transmit desired amounts ofsignal power when communicating with a remote base station withoutexceeding regulatory limits for emitted radiation levels.

The near-field radiation pattern smoothing structures may includestructures such as parasitic antenna resonating element 66. Ferrite tape74 may also help to reduce hotspots and/or near-field signal intensitieswhile allowing desired far-field antenna efficiency criteria to besatisfied. Proximity-sensor-based adjustments may be used in conjunctionwith these techniques if desired.

Parasitic antenna resonating element 66 may be formed from one or moreconductive structures. For example, parasitic antenna resonating element66 may be formed from a rectangular (patch) structure, a straight orbend elongated structure, a structure with a notch, a structure with acurve, other suitable shapes, and combinations of these shapes. Some orall of these structures may serve as capacitive proximity sensorelectrodes.

FIG. 8 is a top view of parasitic antenna resonating element 66 in whichthe parasitic antenna resonating element is formed from a substantiallyrectangular conductive member (e.g., a rectangular patch). The patch mayhave lateral dimensions of LP and WP. Any suitable sizes may be used fordimensions LP and WP if desired. As an example, LP may be about 40 mm(e.g., 10-70 mm) and WP may be about 15 mm (e.g., about 5-25 mm). Theoutline of antenna window 58 may also be rectangular and may have anysuitable dimensions. For example, the outline of antenna window 58 mayhave lateral dimensions of L and W. With one suitable arrangement, L maybe about 80 mm (e.g., 50-110 mm) and W may be about 15 mm (e.g., about5-25 mm).

Capacitor 124 may be coupled between housing 12 (e.g., the antennaground) and parasitic antenna resonating element 66 using capacitorterminals 126 and 128. The capacitance of capacitor 124 may be selectedto provide sufficient coupling between terminal 126 and terminal 128 andtherefore housing 12 and element 66 at the operating frequencies ofantenna 26 (e.g., at 850-2100 MHz, as an example). For example, thecapacitance of capacitors such as capacitor 124 may be about 1-5 pF(i.e., less than 100 pF).

Parasitic antenna resonating element 66 may serve as part of acapacitive proximity sensor. With this type of arrangement, element 66may serve to transmit and receive radio-frequency signals (e.g., atsignals frequencies of 850 MHz and above), while simultaneously servingas a capacitor electrode at lower frequencies (e.g., at frequencies ofabout 200-250 kHz, at frequencies below 1 MHz, or other suitablefrequencies). At these lower frequencies, the circuitry of proximitysensor 80 (FIG. 5) may detect changes in capacitance as an externalobject nears the capacitor electrode.

An illustrative capacitive proximity sensor arrangement that may be usedfor proximity sensor 80 of FIG. 5 is shown in FIG. 9. As shown in FIG.9, proximity sensor 80 may include control circuitry such as signalgenerator 130 and signal detector 132. Conductive element 66 may serveas an electrode for proximity sensor 80. Signal generator 130 may be,for example, a voltage source that produces an alternating current (AC)signal at a frequency of about 200-250 kHz (as an example). Signaldetector 132 may be a current meter or other suitable measurementcircuit for monitoring signals associated with capacitor electrode 66.

During operation, signal detector 132 can monitor the capacitanceassociated with electrode 66. When a user's leg or other external object87 comes within range of electrode 66, the presence of the externalobject will create a change in capacitance that can be detected bysignal detector 132. Signal detector 132 can provide an output signal online 134 that is indicative of the presence or absence of externalobject 87 in the vicinity of electrode 66. This signal, which may beprovided in analog or digital form, may be a Boolean value that has afirst logic value (e.g., a logic zero) when external object 87 is notdetected and that has a second logic value (e.g., a logic one) whenexternal object 87 is detected.

The output signal on line 134 may also have a level that variescontinuously in response to different detected capacitance changes. Withthis type of arrangement, proximity detector 80 may estimate the valueof the distance D that separates electrode 66 from external object 87.When object 87 is close, the proximity detector will produce arelatively high value on output 134. When object 87 is far, theproximity detector will produce a relatively low value on output 134.The signal on output 134 may be an analog signal (e.g., an analogvoltage) or a digital value.

The output signal on path 134 may be fully processed (e.g., to indicatethe value of D) or may be a raw signal (e.g., a signal that representsthe detected capacitance value from electrode 66). Raw signals may beprocessed further using storage and processing circuitry 16. Otherarrangements may be used if desired. For example, other signal sourcesmay be used, other signal detecting schemes may be used, signal outputsmay be provided using a combination of analog and digital signals, etc.

Sensor electrode 66 may be formed from any suitable conductivestructures that can detect capacitance changes due to the presence of anexternal object such as a human body part. The shape of electrode 66when viewed from the top may have straight sides, curved sides, mixturesof straight and curve sides, or other suitable shapes. For example,electrode 66 may have a rectangular outline. The dimensions of electrode66 may be such that the outline of electrode 66 fits within the outlineof dielectric antenna window 58, as shown in FIG. 8. In cross-section,the thickness of electrode 88 may be less than 1 mm, less than 0.5 mm,less than 0.2 mm, less than 0.1 mm, or any other suitable thickness.Substrates such as rigid and flexible printed circuit board substratesmay be used in forming electrode 66. Electrode 66 may also be formedfrom metal foil or other conductive materials.

Electrode 66 may be formed from a single layer of conductive material ortwo or more layers of conductive material. For example, electrode 66 maybe formed from a flex circuit substrate or other printed circuit boardsubstrate having an upper conductive layer and a lower conductive layer.The upper and lower layers may be, for example, rectangular conductivetraces formed on a flex circuit or rigid printed circuit boardsubstrate. The conductive traces may be formed from a metal such ascopper.

With this type of two layer arrangement, one of the electrode layers mayserve as a sensor electrode layer and the other of the electrode layersmay serve as an active shield layer. An illustrative arrangement of thistype is shown in FIG. 10.

As shown in FIG. 10, sensor electrode 66 may have upper layer 66A andlower layer 66B. Lower layer 66B may be a sensor electrode layer(sometimes referred to as a sensor electrode). Upper layer 66A may be anactive shield layer (sometimes referred to as an alternating currentshield or AC shield).

Capacitances that are associated with a capacitive sensor configurationthat uses a two-layer sensor electrode are showing in FIG. 10. Theconductive layers in sensor electrode 66 may be coupled to signaldetector 132. In the example of FIG. 10, signal detector 132 includes acapacitance-to-digital converter (CDC) 136 that is connected toelectrode layers 66A and 66B through respective inductors L2 and L1.Inductors L1 and L2 may have inductance values of about 220-390 nH(e.g., 390 nH) or other suitable values that allow inductors L1 and L2to serve as radio-frequency chokes (i.e., radio-frequency chokeinductors). Radio-frequency signals that are transmitted by antennaresonating element 68 can be electromagnetically coupled into theconductive structures of sensor electrode 66. When the inductance valuesof L1 and L2 are selected properly, these radio-frequency signals aresubject to a relatively high impedance and are not passed tocapacitance-to-digital converter 135. At the same time thatradio-frequency signals from antenna resonating element 68 are beingblocked by inductors L1 and L2 (which serve as radio-frequency chokes),lower frequency signals such as alternating current (AC) excitationsignals in the kHz range that are supplied to sensor electrode 66 bysource 130 (FIG. 9) can pass from sensor electrode 66 tocapacitance-to-digital converter through inductors L1 and L2. This isbecause the impedances of inductors L1 and L2 scale with frequency.

Capacitance-to-digital converter 136 may be implemented using anysuitable capacitive touch sensor control circuit. With one suitablearrangement, capacitance-to-digital converter 136 may be implementedusing the AD7147 programmable capacitance-to-digital converterintegrated circuit available from Analog Devices of Norwood, Mass.Capacitance-to-digital converter 136 converts a capacitive input signalon its input to a digital capacitance value on its output.

During operation, the measured capacitance C2 between conductiveelectrode layers 66A and 66B can be minimized by driving signals ontoconductors 66A and 66B in parallel. This helps to improve sensorperformance. There is typically a fixed capacitance C1 of about 150 pFor less between sensor electrode 66A and housing 12. Capacitance C1arises from electromagnetic fields within housing 12 and is notresponsive to changes in the position of external object 87 with respectto electrode 66. Fringing electric fields outside of housing 12 giverise to a capacitance CA between conductive layer 66B and housing 12.Variable capacitance CAX arises between external object 87 andconductive layer 66B. The magnitude of capacitance CAX depends on thedistance between external object 87 and electrode layer 66B. Whenexternal object 87 is not present, the value of CAX is at a minimum. Asobject 87 approaches layer 66B, the value of CAX rises. Relatively largevalues of CAX arise when object 87 is in the vicinity of layer 66B(i.e., when object 87 is less than 2 cm or other suitable distance fromlayer 66B. Capacitance-to-digital converter 136 can measure capacitanceCAX (which is in parallel with capacitance CA) and can produce acorresponding digital capacitance value. Storage and processingcircuitry 16 (FIG. 3) may receive the digital capacitance value that hasbeen measured by capacitance-to-digital converter 136 and can compute acorresponding distance value that is indicative of the distance ofexternal object from sensor electrode 66.

When external object 87 is in proximity to sensor electrode 66 (e.g.,when a user places device 10 on the user's lap so that antennaresonating element 68 and other structures in antenna 26 are close tothe user's leg), capacitance-to-digital converter (CDC) 136 can output acorrespondingly high capacitance value. Storage and processing circuitry16 can analyze the capacitance signal from capacitance-to-digitalconverter 136 and can take appropriate action.

For example, if storage and processing circuitry 16 concludes thatexternal object 87 is more than 2 cm (or other suitable distance) fromantenna resonating element 68 and other such antenna structures indevice 10, transceiver circuitry 23 can be allowed to transmitradio-frequency antenna signals at any desired power including themaximum available transmit power for device 10. If, however, storage andprocessing circuitry 16 concludes that external object 87 is in thevicinity of antenna 26, storage and processing circuitry 16 can limitthe amount of permissible transmit power from transceivers 23. In thisway, storage and processing circuitry 16 can use external objectproximity information in determining what radio-frequency output powerlevel to use in operating transceiver circuitry 23. When an externalobject such as a user's body is close to device 10 and antenna 26, themaximum transmit power can be reduced to ensure compliance withregulatory limits. When no external object is in the vicinity of device10 and antenna 26, proximity-based transmit power limits may be removedand larger radio-frequency output powers can be used.

Illustrative configurations that may be used for a two-layer sensorelectrode are shown in FIGS. 11 and 12. As shown in FIG. 11, capacitivesensor electrode 66 may have conductive layers 66A and 66B that areformed from conductive traces on opposing sides of dielectric substrate138. The outline of layers 66A and 66B may be rectangular (as shown inFIG. 11) or may have other suitable shapes. Capacitor 124 (FIG. 8) maybe connected to layer 66A at terminal 128 (as an example). Dielectricsubstrate 138 may be plastic, epoxy (e.g., fiberglass-filled epoxy suchas FR4 or other rigid printed circuit board dielectrics), or a flexiblepolymer sheet (e.g., a polyimide layer for a flex circuit). Conductivelayers 66A and 66B may be formed by physical vapor deposition,electroplating, screen printing, or any other suitable layer formationtechnique. Layers 66A and 66B may be less than 0.1 mm thick, less than0.05 mm thick, less than 0.01 mm thick, etc. Dielectric substrate layer138 may be less than 1 mm thick, less than 0.5 mm thick, less than 0.1mm thick, less than 0.05 mm thick, etc.

In the illustrative layout of FIG. 11, sensor electrode 66 has asubstantially rectangular outline. If desired, sensor electrode 66 mayhave non-rectangular shapes. As shown in FIG. 12, for example, sensorelectrode 66 may have an elongated shape with one or more bends. In FIG.12, sensor electrode 66 has three layers: conductive layer 66A,dielectric layer 138, and conductive layer 66B. If desired, electrode 66may have more layers or fewer layers. Layers 66A and 66B may be metallayers or layers of other suitable conductive materials, as described inconnection with FIG. 11. Layer 138 may be a printed circuit boardsubstrate such as a rigid printed circuit board or a flex circuitsubstrate. As with dielectric substrate layer 138 of FIG. 11, typicalthicknesses that may be used for substrate 138 are less than 1 mm. Forexample, dielectric layer 138 may be less than 0.5 mm thick, less than0.1 mm thick, less than 0.05 mm thick, etc.

The layouts of FIGS. 11 and 12 are merely illustrative. Any suitablesensor electrode layout may be used if desired. Sensor electrode 66 may,for example, have elongated shapes, shapes with straight sides, shapeswith curved sides, etc. Single-layer arrangements and multi-layerarrangements may be used. As described in connection with FIGS. 6-8,sensor electrode 66 may serve as a parasitic antenna resonating elementthat reduces radio-frequency hotspots in the electromagnetic radiationemitted by device 10. This may help ensure that device 10 satisfiesregulatory limits for radio-frequency signal transmission powers,particularly through the lower portion of device 10 where device 10 maycome into contact with an external object such as the human body.

The foregoing is merely illustrative of the principles of this inventionand various modifications can be made by those skilled in the artwithout departing from the scope and spirit of the invention.

What is claimed is:
 1. An electronic device, comprising: a housing; anantenna window in the housing; an antenna resonating element mounted inthe housing so that radio-frequency signals are transmitted through theantenna window; and a capacitive proximity sensor electrode locatedbetween the antenna resonating element and the antenna window.
 2. Theelectronic device defined in claim 1 wherein the capacitive proximitysensor comprises: a dielectric layer; and first and second conductivelayers on opposing sides of the dielectric layer.
 3. The electronicdevice defined in claim 2 wherein the dielectric layer comprises aflexible sheet of polymer.
 4. The electronic device defined in claim 3wherein the first and second conductive layers comprise rectangles ofmetal.
 5. The electronic device defined in claim 2 wherein thedielectric layer comprises a rigid printed circuit board substrate. 6.The electronic device defined in claim 2 wherein the housing comprises aconductive housing, the electronic device further comprising a capacitorconnected between first conductive layer and the conductive housing. 7.The electronic device defined in claim 2 wherein the conductive housingcomprises a metal housing, the electronic device further comprising: apositive antenna feed terminal connected to the antenna resonatingelement; a ground antenna feed terminal connected to the metal housing;and a capacitor connected between the metal housing and the capacitiveproximity sensor electrode, wherein the capacitive proximity sensorelectrode serves as a parasitic antenna resonating element.
 8. Theelectronic device defined in claim 7 wherein the electronic device hasfront and rear surfaces, the electronic device further comprising adisplay on the front surface of the electronic device, wherein thedisplay has an inactive region through which radio-frequency signals aretransmitted from the antenna resonating element.
 9. The electronicdevice defined in claim 7 further comprising ferrite tape between thecapacitive proximity sensor electrode and the antenna window.
 10. Theelectronic device defined in claim 1 further comprising: acapacitance-to-digital converter having first and second inputs; andfirst and second radio-frequency choke inductors coupled between thecapacitive proximity sensor electrode and the capacitance-to-digitalconverter.
 11. The electronic device defined in claim 10 wherein thecapacitive proximity sensor electrode comprises: a dielectric layer; andfirst and second conductive layers on opposing sides of the dielectriclayer, wherein the first conductive layer is connected to the firstinput by the first radio-frequency choke inductor and wherein the secondconductive layer is connected to the second input by the secondradio-frequency choke inductor.
 12. The electronic device defined inclaim 11 further comprising a display having display panel circuitrythat is covered by a transparent dielectric cover member, wherein theantenna resonating element emits radio-frequency signals that passthrough the transparent dielectric cover member without passing throughthe display panel circuitry.
 13. A tablet computer comprising: aconductive housing; a dielectric antenna window in the conductivehousing; radio-frequency transceiver circuitry; an antenna with whichthe radio-frequency transceiver circuitry transmits radio-frequencysignals in at least one cellular telephone band, wherein the antennacomprises an antenna ground formed from at least portion of theconductive housing and an antenna resonating element mounted adjacent tothe dielectric antenna window; and a capacitive proximity sensorelectrode mounted between the antenna resonating element and thedielectric antenna window.
 14. The tablet computer defined in claim 13further comprising a capacitance-to-digital converter coupled to thecapacitive proximity sensor electrode.
 15. The tablet computer definedin claim 14 wherein the capacitive proximity sensor comprises first andsecond conductive layers separated by a dielectric layer.
 16. The tabletcomputer defined in claim 15 further comprising a pair of inductorscoupled between the capacitive proximity sensor electrode and thecapacitance-to-digital converter.
 17. The tablet computer defined inclaim 13 further comprising a capacitor having a first terminalconnected to the conductive housing and a second terminal connected tothe capacitive proximity sensor electrode, wherein the capacitiveproximity sensor electrode serves as a parasitic antenna resonatingelement for the antenna.
 18. An electronic device comprising: at leastone conductive housing structure to which a ground antenna feed terminalis connected; an antenna window in the housing structure; an antennaresonating element formed from conductive traces on a flex circuit towhich a positive antenna feed terminal is connected; radio-frequencytransceiver circuitry that is coupled to the positive antenna feedterminal and the ground antenna feed terminal and that transmitsradio-frequency signals through the antenna window using the antennaresonating element; and a capacitive proximity sensor electrodeinterposed between the antenna resonating element and the antennawindow.
 19. The electronic device defined in claim 18 wherein theradio-frequency transceiver circuitry transmits the radio-frequencysignals at an output power, the electronic device further comprising:circuitry coupled to the capacitive proximity sensor electrode thatlimits the output power when an external object is detected within agiven distance of the capacitive proximity sensor electrode.
 20. Theelectronic device defined in claim 19 wherein the capacitive proximitysensor electrode comprises first and second conductive layers coupled tothe circuitry by respective first and second inductors and wherein thecircuitry comprises a capacitance-to-digital converter that makescapacitance measurements on the capacitive proximity sensor electrode.21. The electronic device defined in claim 18 wherein the capacitiveproximity sensor electrode serves as a parasitic antenna resonatingelement, the electronic device further comprising a capacitor connectedbetween the conductive housing structure and the capacitive proximitysensor electrode.
 22. The electronic device defined in claim 21 whereinthe capacitive proximity sensor comprises first and second conductivelayers separated by a dielectric substrate.